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Encyclopedia > Ephemeris time

Ephemeris Time (ET) is a now obsolete time scale used in ephemerides of celestial bodies, in particular the Sun (as observed from the Earth), Moon, planets, and other members of the solar system. This is distinct from Universal Time (UT): the time scale based on the rotation of the Earth around its axis. ET was replaced with the two time scales Terrestrial Dynamical Time (TDT) and Barycentric Dynamical Time (TDB) by the International Astronomical Union (IAU) in 1976. TDT was renamed Terrestrial Time (TT) in 1991. An ephemeris (plural: ephemerides) (from the Greek word ephemeros= daily) was, traditionally, a table providing the positions (given in a Cartesian coordinate system, or in right ascension and declination or, for astrologers, in longitude along the zodiacal ecliptic), of the Sun, the Moon, and the planets in the sky at... Universal Time (UT) is a timescale based on the rotation of the Earth. ... This article is about terrestrial time; for other meanings of TT, see TT (disambiguation). ... Barycentric Dynamical Time (TDB) was defined by the International Astronomical Union (IAU) in 1976 to be used as the relativistic replacement for the non-relativistic Ephemeris Time which had been used in the ephemerides starting in 1960. ... Logo of the IAU The International Astronomical Union (French: Union astronomique internationale) unites national astronomical societies from around the world. ... Terrestrial Time (TT) is the modern time standard for time on the surface of the Earth. ...

In the late nineteenth century it was found that the rotation of the Earth (i.e. the length of the day) was both irregular on short time scales, and was slowing down on longer time scales. In fact, observing the position of the Moon, Sun and planets and comparing this with their ephemerides was a better way to determine the time. Water, Rabbit, and Deer: three of the 20 day symbols in the Aztec calendar, from the Aztec Sun Stone. ...

Using the ephemerides based on the theory of the apparent motion of the Sun by Simon Newcomb (1898), the SI second was defined in 1960 as: Newcombs Tables of the Sun is the short title for a work by the American astronomer and mathematician Simon Newcomb entitled Tables of the Motion of the Earth on its Axis and Around the Sun on pages 1-169 of volume VI of the serial publication Astronomical Papers prepared... Simon Newcomb. ... Cover of brochure The International System of Units. ... Look up second in Wiktionary, the free dictionary. ...

the fraction 1/31,556,925.9747 of the tropical year for 1900 January 0 at 12 hours ephemeris time.

Caesium atomic clocks became operational in 1955, and quickly made it evident that the rotation of the earth fluctuated randomly. This confirmed the utter unsuitability of the mean solar second of Universal Time as a measure of time interval. After three years of comparisons with lunar observations it was determined that the ephemeris second corresponded to 9,192,631,770 cycles of the caesium resonance. Between 1960 and 1984 the length of the SI second was defined to be equal to the ephemeris second. A tropical year is the length of time that the Sun, as viewed from the Earth, takes to return to the same position along the ecliptic (its path among the stars on the celestial sphere). ... General Name, Symbol, Number caesium, Cs, 55 Chemical series alkali metals Group, Period, Block 1, 6, s Appearance silvery gold Atomic mass 132. ... An atomic clock is a type of clock that uses an atomic resonance frequency standard as its counter. ...

As the theoretical basis for Ephemeris Time is wholly non-relativistic, in 1976 the IAU resolved that beginning in 1984 ET would be replaced by the two relativistic timescales Barycentric Dynamical Time (TDB) and Terrestrial Dynamical Time (TDT). For practical purposes the length of the ephemeris second can be taken as equal to the length of the TDB or TDT second.

The difference between ET and UT is called ΔT; it changes irregularly, but the long-term trend is parabolic, decreasing from ancient times until the nineteenth century, and has been increasing at about 0.7 seconds per year since (see leap seconds). International Atomic Time (TAI) was set equal to UT2 at 1 January 1958 0:00:00 . At that time, ΔT was already about 32.18 seconds. The difference between Terrestrial Time (TT) (the successor to ephemeris time) and atomic time was later defined as follows: Delta T and delta-T are ASCII substitutes for the formal ΔT, which is Terrestrial Time minus Universal Time. ... A parabola The parabola (from the Greek: παραβολή) is a conic section generated by the intersection of a right circular conical surface and a plane parallel to a generating straight line of that surface. ... A leap second is a one-second adjustment to civil time in order to keep it close to the mean solar time. ... International Atomic Time (TAI, from the French name Temps Atomique International) is a high-precision atomic time standard that tracks proper time on Earths geoid. ... Universal Time (UT) is a timescale based on the rotation of the Earth. ...

1977 January 1.0003725 TT = 1977 January 1.0000000 TAI, i.e.
ET - TAI = 32.184 seconds

This difference may be assumed constant—the rates of TT and TAI are designed to be identical.


  • P.K.Seidelmann (ed.), Explanatory Supplement to the Astronomical Almanac. University Science Books, CA, 1992 ; ISBN 0-935702-68-7

  Results from FactBites:
The Time (0 words)
Before even the ephemeris second was defined, the second (not “SI” because this is before SI) was defined as the fraction 1/86400 of the mean solar day (this is, after all, the historical definition of the second, the day being divided in 24 hours of 60 minutes of 60 seconds each).
More precisely, it is the time coordinate of a space-time coordinate system that is centered in space on the barycenter of the solar system, that is nonrotating with respect to distant galaxies, and that tends asymptotically to the proper time of an observer at rest with respect to the coordinate system.
Geocentric Coordinate Time, or TCG, is the same as Barycentric Coordinate Time, except that it uses the Earth's center, rather than the barycenter of the solar system (and an “asymptotic” observer is one free from the gravitational field of the Earth rather than that of all the solar system).
Astronomical Time Keeping (0 words)
Moreover, siderial time is one of the constituents of Universal Time.
The Ephemeris Time (ET) was consequently defined as the timescale that together with the laws of motion correctly predicts the positions of celestial bodies, and it is therefore used as the argument in the ephemerides.
Ephemeris time is reckoned from the instant, near the beginning of the calendar year AD 1900, when the geometric mean longitude of the Sun was 279 degrees, 41 minutes 48.04 seconds, at which instant the measure of Ephemeris Time was 1900 January 0, 12 o'clock precisely.''
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